In addition to masking the later packages, I used the invocation suggested in another thread and got the desired result:

># emerge -av1 =xorg-server-1.12.4

For those unfamiliar with the syntax, the -1 (it's a numeral 1) means 'one-shot', and doesn't include the package in 'world' for subsequent upgrades, and the '=' is an operator specifying a package version rather than the default (latest). The -av is 'ask' and 'verbose' - `man emerge` describes them.